If you have issues with incoming or outgoing calls you probably need to downgrade the DECT module software of the 300Wx.
To check this, go to, MENU -> Status and then cycle with the arrow keys to find FW932 or similar. 
You need version 925 or earlier.

An SD card is needed.
Put upgrade.fwu (https://upgrade.konftel.com/download/konftel_300wx_dect/925/upgrade.fwu) on the SD-card.

Go into the service menu by pressing MENU, long press (2 seconds) on HOLD, long press (2 seconds) on Connectivity button.*
Select DECT upgrade with the SD-card in the unit.
Now reboot the unit (Take the battery out and back in again).
Check the DECT firmware, MENU -> Status and cycle with the arrows, FWVERSION 925 should be the DECT firmware.
Register if it is not yet registered.*